Transcript Electric Field

Chapter 23 Electric Fields
23.1 Properties of Electric Charges
•
•
•
•
•
•
Two kinds + and –
Charge is a scalar
Like charges repel and unlike attract
Conductor: charges relatively free to move
Insulator: charges relatively fixed
Charge is conserved in an isolated system – you can
transfer charge, but not create or destroy charge
• The MKS unit of charge is 1 Coulomb
• Charge is quantized: e = 1.6 x 10-19 Coulombs.
Typically there are around 1013 excess or deficient
charges on a body – 1013/1023 or 1 part in 1010
23.2 Induced Charges
CT1: Two uniformly charged spheres are firmly
fastened to and electrically insulated from
frictionless pucks on an air table. The charge on
sphere 2 is three times the charge on sphere 1.
Which force diagram correctly shows the
magnitude and direction of the electrostatic forces:
A.
D.
B.
E.
C.
F.
G. None of these.
23.3 Coulomb’s Law
F12 = keq1q2/r2 r^
ke = 8.99 x 109 Nm2/C2
CT2: P23.5
A hydrogen atom is composed of a nucleus
containing a single proton, about which a
single electron orbits. The electric force
between the two particles is 2.3 x 1039
greater than the gravitational force! If we
can adjust the distance between the two
particles, can we find a separation at which
the electric and gravitational forces are
equal?
A. Yes, we must move the particles
farther apart.
B. Yes, we must move the particles
closer together.
C. No, not at any distance
Chapter 23 Electric Fields
23.3 Coulomb’s Law
Superposition
The effect of several electrical forces acting on
a charge due to several other charges is the
vector sum of the individual forces from each
charge as if the remaining charges weren’t
present.
F1 = F21 + F31 + F41 + F51 + F61 + F71 + …..
P23.5 (p.666)
P23.8 (p.667)
CT3: P23.8
Which statement is true?
A. The charged bead is in stable
equilibrium only if the charge is
positive.
B. The charged bead is in stable
equilibrium only if the charge is
negative.
C. The charged bead is in stable
equilibrium if the charge is
either positive or negative.
D. The charged bead is in never in
stable equilibrium.
Chapter 23: Electric Fields
Ch23.4 The Electric Field
I. Definition
Coulomb Force: charge1
Electric Field: charge1
E
=
lim F/q0
q
0
0
charge2
electric
field
charge2
q0 is a positive
test charge
Chapter 23: Electric Fields
Ch23.4 The Electric Field
II. Calculation of E
A. Point Charge
B. Discrete Distribution of Point Charges
Discrete sum – superposition (P 23.17 and P23.18)
Note: E depends only on the source charge(s)
CT4: P23.18
For very large x
A. The dipole field is stronger than
the monopole (single charge)
field.
B. The dipole field is weaker than
the monopole field.
C. The dipole field is equal to the
monopole field.
Ch23.5 Electric Field of a
Continuous Charge Distribution
Continuous Distribution (P23.27)


∫
Uniform volume charge density
 = q/V and dq = dV
Uniform surface charge density
 = q/A and dq = dA
Uniform linear charge density
 = q/l and dq = dl
Note: E depends only on the source charge distribution
dl = rd

dq = dl
r
dEx
dE
Ch23.27
Ch23.6 Electric
Field Lines
• Electric field lines give a visualization
of the electric force field
• The field points along the line in the
direction of the arrow
• Field lines leave + charges and end on
– charges
• The number of lines is proportional to
the source charge, so the density of
lines is proportional to field strength
Ch23.6 Electric Field Lines
Electric Field Lines – Isolated Positive Point Charge
Electric Field Lines – Isolated Negative Point Charge
Electric Field Lines – Isolated Positive and
Negative Point Charges Compared
Electric Field Lines – Two Isolated and
Separated Equal Positive Point Charges
Electric Field Lines – Isolated and
Separated Equal Positive and Negative
Point Charges – Electric Dipole
Electric Field Lines – Two Parallel Oppositely, but Equally,
Charged Plates - Capacitor
Electric Field Lines – Two Isolated Separated
Charges of Opposite Sign and Different Magnitudes
CT23.5
CT23.6
• 1
A. +,+
• 2
B. +,-
• 3
C. -,+
• 4
D. -,-
P23.40 a) What is the ratio q2/q1? b) Signs of q1 and q2?
Ch23.7 Motion of Charged
Particles in a Uniform Electric
Field
Squeak Demo
Do P23.41
The coin doesn’t have to be charged for the cup to attract the
coin. The charged cup can induce a polarization of the charge
which results in an attraction.
Common misconceptions - Charged objects only interact
with other charged objects. Charged objects do not interact
with uncharged (neutral) objects. Uncharged (neutral) is
considered to be a third type of charge. Charged and
uncharged objects attract each other because they are
opposites.
Correct answer